The invention relates generally to apparatus and methods for igniting air/fuel mixtures in combustors. More particularly, the invention relates to the use of infrared radiation in such apparatus and methods.
The combustor is a fundamental assembly used in turbine and other engines. The combustor typically includes a can or other annular casing that forms part or all of a combustion chamber for the fuel. Within the combustor are one or more fuel nozzles which deliver fuel to the combustion chamber, along with air vents for delivering high pressure air to the combustion chamber. The air/fuel mixture is ignited in or near the region of the combustor closest to the fuel nozzles (i.e. the primary zone). The combustion process continues as the combusting fuel/air mixture moves down to the intermediate zone where additional air is supplied to cool the combustor wall and aid the combustion process. The process continues as the mixture of hot combustion gases enters the dilution zone where dilution air is supplied to cool the exhaust gases to protect the annulus casing from melting and downstream to protect the turbine blades. As is well known, homogeneity of the fuel burn within the combustion chamber is an important design criteria for a turbine engine.
Fuel delivery systems play an important part in the ability to initiate or restart a turbine engine. In known combustors, the fuel nozzles typically include a primary orifice and one or more secondary orifices. The purpose of the nozzle primary orifice is to provide initially a finely atomized fuel spray that can be ignited for engine start. After combustion starts and the engine speed increases, the secondary orifices are opened to increase fuel flow.
The ease with which fuel can be ignited in the combustor depends on several key factors including fuel temperature, the type of igniter used, amount of ignition energy delivered, point of ignition energy delivery and the degree to which the fuel is atomized by the nozzle via the primary orifice. The atomization process is also important with respect to the overall efficiency of the fuel combustion.
Combustors require an igniter device to initiate the combustion process. Known igniters are plasma type spark plugs and glow plugs. Typically, the spark plug is mounted in the combustor wall near the fuel nozzle. In a conventional combustor, the primary zone or optimum region for ignition is the high turbulence region just forward of the nozzle outlet. However, the igniter cannot protrude down into this optimum region because it would be destroyed by the fuel combustion process. Retractable igniters are sometimes used with furnaces, but are not deemed reliable for aerospace applications. Thus, particularly in aircraft engine combustors, the igniter is mounted in a recess on the wall of the combustor near the primary zone. A high energy plasma, high temperature spark kernel is created at the periphery of the combustor wall and protrudes into the combustion chamber. However, there are numerous disadvantages including the fact that the combustor wall tends to act as a heat sink and quenches the intensity of the spark. The fuel/air mixture also is not optimum in this region. Obviously, the combustors are designed so that this type of ignition arrangement works, but it is less than ideal.
A known alternative to the spark kernel is the use of a torch burner which creates a flame that is used to ignite the main fuel supply in the primary zone of the combustion chamber. Known torch burners, however, still produce less than ideal results because of their reliance on conventional fuel supply nozzles and orifices.
Conventional plasma type spark plugs are commonly used for igniters. Unfortunately, by their very nature of using high voltage/current plasma discharge, they exhibit considerable electrode degradation and must be routinely replaced. Also, less than optimum combustion, particularly during engine start up and shut down, and/or fuel exposure, can produce plug fouling which degrades the spark discharge intensity or can prevent ignition. Varnish and other combustion by-products, particularly due to incomplete combustion and fuel evaporation, also can deteriorate plug performance. As a result, very high energy levels must be delivered to the spark plug to insure that carbon and fuel deposits are literally blown off the electrodes to produce an adequate spark. This excess energy, however, causes more rapid degradation of the electrodes, thereby shortening their useful life and increasing maintenance. Furthermore, the high energy required to produce the spark is typically supplied from an exciter circuit, such as a capacitive or inductive discharge exciter. The exciter circuit is located remote from the combustion chamber, however, due to the associated electronics. Consequently, the exciter must be connected to the plug by way of long coaxial cable leads or wires. This wiring causes many problems, not the least of which is simply energy loss. For example, to produce a two joule discharge at the plug, the exciter circuit may be required to output ten joules of energy into the ignition coaxial cable leads, resulting in low ignition system efficiencies, hence higher weight and cost.
The conventional plasma spark plug type igniters have another drawback in that the use of an exciter to produce high discharge voltages typically also requires the use of high tension leads or conductors. At high operating altitudes these leads can exhibit a voltage break down--i.e. corona. Therefore, such ignition systems require careful design including pressure sealing to prevent corona effects at higher altitudes. These design requirements add to the cost of such systems as well as may contribute to less than ideal performance.
An alternative ignition technique has been disclosed in U.S. Pat. No. 4,947,640 issued to Few et al. This patent describes a combustion ignition technique that uses ultraviolet laser energy. The laser energy is directed into the combustion chamber and is absorbed by the atomized fuel droplets. The ultraviolet wavelength region is used because the hydrocarbon fuels commonly used in engines and combustors absorb this wavelength of electromagnetic energy to a high degree. This absorption of the laser energy can sufficiently heat the fuel molecules to produce vaporization and combustion. The wavelength of the laser energy, however, needs to be fine tuned to the optimum wavelength for efficient absorption to produce combustion. Enough energy must be absorbed to create free radicals and oxygen dissociation.
The need to fine tune the operating wavelength of the ultraviolet laser energy is a significant drawback to the aforementioned system. For example, in aerospace technology, and in particular aircraft fuels, the fuel includes various additives and inhibitors (such as, for example, to improve low temperature operating performance to increase fuel conductivity or to reduce soot.) These additives can change the absorption characteristics of the fuel, thus requiring specific wavelength compensation depending on the chemical composition of the fuel at the exact time ignition is required or the redesign of the laser system each time a fuel formulation is modified. This is not, from a practical standpoint, very useful for ensuring ignition and more importantly relight after a flame out condition occurs. Other laser chemistry induced reactions have been investigated. For example, see U.S. Pat. No. 4,343,687.
Another substantial problem is that the combustion by-products that are produced, such as varnish and soot, also are highly absorptive of ultraviolet radiation. This residue most frequently occurs during engine startup and shutdown because during these periods the combustion temperature is lower thus resulting in incomplete combustion of the fuel and additives. Also, exhaust flow is reduced during these periods. As a result, a significant amount of varnish and soot can build up inside the combustor, and in particular will adhere to any surface such as the combustor liner or an optical window in the liner for the ultraviolet laser energy to pass through, and this residue will absorb the very ultraviolet light that is trying to be used to ignite the fuel. Thus, ignition might be inhibited altogether, or the laser power would have to be unacceptably high to ensure ignition.
Another shortcoming of the known laser ignition systems is that they do not take into account the fact that the system should be capable of a direct replacement retrofit into existing systems. Not only would this allow replacement in the field, but this would also not necessitate, in and of itself, a redesign of the combustor. For example, laser augmentation for afterburners has been suggested in U.S. Pat. No. 4,302,933 (issued to Smith), but such a system is impractical from a retrofit approach.
Laser energy has also been suggested for chemical reaction processes in a closed chamber, and for rocket motor combustion. See, for example, U.S. Pat. Nos. 4,666,678 and 4,702,808 issued to Lemelson. These systems, however, require highly controlled and regulated air/fuel supplies such as through valve arrangements or similar means. Such apparatus, therefore, are impractical for flow through type combustors. These systems also specify high energy laser sources (current laser systems utilized for welding and chemical destruction are considered to be high energy laser sources on the order of 1.5 kw to 10 kw), again rendering such apparatus impractical for aircraft combustor applications.
The need exists, therefore, for better and more reliable and more efficient apparatus and methods for initiating combustion, particularly for engine restart. There also is a need for an improved igniter that does not have the problems associated with conventional plasma type plugs. As a practical matter, such an igniter and ignition system should be capable of a direct retrofit replacement for existing plasma plug systems so as to minimize or eliminate the need for combustor designers to redesign the combustors that will use such a new system.